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Prebiotic 'Life' On Earth May Have Cosmic Origin

Origin of Life on Earth: 'Natural' Asymmetry of Biological Molecules May Have Come from Space

ScienceDaily (Jan. 7, 2011) — Certain molecules do exist in two forms which are symmetrical mirror images of each other: they are known as chiral molecules. On Earth, the chiral molecules of life, especially amino acids and sugars, exist in only one form, either left-handed or right-handed. Why is it that life has initially chosen one form over the other?

A consortium bringing together several French teams led by Louis d'Hendecourt (1), CNRS senior researcher at the Institut d'astrophysique spatiale (Université Paris-Sud 11 / CNRS), has for the first time obtained an excess of left-handed molecules (and then an excess of right-handedones) under conditions that reproduce those found in interstellar space. This result therefore supports the hypothesis that the asymmetry of biological molecules on Earth has a cosmic origin. The researchers also suggest that the solar nebula formed in a region of massive stars.

This work has just been published online on the web site of The Astrophysical Journal Letters. The experiment was carried out at the SOLEIL synchrotron facility in collaboration with the Laboratoire de chimie des molécules bioactives et des arômes (Université de Nice/CNRS) and with the support of CNES.

Chiral molecules are molecules that can exist in two forms (enantiomers) which are symmetrical mirror images of each other, one left-handed and the other right-handed. For instance, our hands are chiral since they come in two forms, the left hand and the right hand, that are symmetrical with their mirror image but not super imposable on it. Biological molecules are mostly chiral, with some forms being favored over others. For instance, the amino acids that make up proteins only exist in one of their two enantiomeric forms, the left-handed (L) form. On the other hand, the sugars present in the DNA of living organisms are solely right-handed (D). This property that organic molecules have of existing in living organisms in only one of their two structural forms is called homochirality.

What is the origin of such asymmetry in biological material? There are two competing hypotheses. One postulates that life originated from a mixture containing 50% of one enantiomer and 50% of the other (known as a racemic mixture), and that homochirality progressively emerged during the course of evolution. The other hypothesis suggests that asymmetry leading to homochirality preceded the appearance of life and was of cosmic origin. This is supported by the detection of L excesses in certain amino acids extracted from primitive meteorites. According to this scenario, these amino acids were synthesized non-racemically in interstellar space and delivered to Earth by cometary grains and meteorites.

To lend more weight to this hypothesis, the researchers first reproduced analogs of interstellar and cometary ices in the laboratory (2). The novel aspect of their experiment was that, using the DESIRS beamline at the SOLEIL synchrotron facility, the ices were subjected to circularly polarized ultraviolet radiation (UV-CPL) (3), which is supposed to mimic the conditions encountered in some space environments. When the ices were warmed up, an organic residue was produced. A detailed analysis of this mixture revealed that it contained a significant enantiomeric excess in one chiral amino acid, alanine. The excess, which was over 1.3%, is comparable to that measured in primitive meteorites. The researchers thus succeeded in producing, under interstellar conditions, asymmetrical molecules of life from a mixture that did not contain chiral substances. This is the first time that a scenario that explains the origin of this asymmetry has been demonstrated using an experiment that reproduces an entirely natural synthesis.

This result reinforces the hypothesis that the origin of homochirality is prebiotic and cosmic, in other words genuinely interstellar. According to this scenario, the delivery of extraterrestrial organic material containing an enantiomeric excess synthesized by an asymmetrical astrophysical process (in this case, UV-CPL radiation) is the cause of the asymmetry of life's molecules on Earth. This material may even have formed outside the solar system. Finally, the solar nebula may have formed in regions of massive star formation. In such regions, infrared radiation circularly polarized in the same direction has been observed.

These findings imply that the selection of a single enantiomer for the molecules of life observed on Earth is not the result of chance but rather of a deterministic physical mechanism.

"Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination." Max Planck

Excellent post mate. I think the possibility that material from space could have played a large role for the development of life on earth. Although there are many methods that life could have formed on earth, each with evidence to support them, one of the more incredible facts of our solar system is that comets, meteorites, and interplanetary dust are LOADED with a wide range of organic compounds, including amino acids, hydrogen cyanide, formaldehyde, adenine, and many others. The amount of organic material currently deposited on the earth by meteorites and interplanetary dust is estimated to be about three hundred thousand kilograms, or 300 metric tones per year. During the early period of heavy bombardment this rate may have reached 50,000 tons per year. This rate would have produced the current total biomass in approximately ten million years.

Comets are the richest source of organic compounds. This organic material makes its way to earth in the form of micrometeorites, which are formed when the comets pass close to the sun. These micrometeorites are 50 to 500 um in size and currently reach the earth in huge amounts of 20,000 tons per year. They contain tiny grains that contain clays, oxides, and sulfides of metals, which can act as catalysts in chemical reactions.

Based on all of this, I am not surprised that we find similarities in our biological molecules that resembles those found in space. What creationists love to play is the 'chance game' but what they fail to understand is that life is not a one shot thing, and the calculate it as occurring one time instead of a pool of millions of organic compounds forming over and over repeatably for millions of years. The creationists say life is a miracle, but I think the real miracle would be if life did not form on this planet, or any planet, at all.